Fix compiler warning in EXEC_BACKEND builds.

Per a report by Rajeev Rastogi.
Allow using huge TLB pages on Linux (MAP_HUGETLB)

This patch adds an option, huge_tlb_pages, which allows requesting the
shared memory segment to be allocated using huge pages, by using the
MAP_HUGETLB flag in mmap(). This can improve performance.

The default is 'try', which means that we will attempt using huge pages,
and fall back to non-huge pages if it doesn't work. Currently, only Linux
has MAP_HUGETLB. On other platforms, the default 'try' behaves the same as
'off'.

In the passing, don't try to round the mmap() size to a multiple of
pagesize. mmap() doesn't require that, and there's no particular reason for
PostgreSQL to do that either. When using MAP_HUGETLB, however, round the
request size up to nearest 2MB boundary. This is to work around a bug in
some Linux kernel versions, but also to avoid wasting memory, because the
kernel will round the size up anyway.

Many people were involved in writing this patch, including Christian Kruse,
Richard Poole, Abhijit Menon-Sen, reviewed by Peter Geoghegan, Andres Freund
and me.
Fix docs build.

Broken by the huge_tlb_pages patch.

Vik Fearing.
Remedy two small omissions in the new json functions docs.
Allow skipping some items in a multi-key GIN search.

In a multi-key search, ie. something like "col @> 'foo' AND col @> 'bar'",
as soon as we find the next item that matches the first criteria, we don't
need to check the second criteria for TIDs smaller the first match. That
saves a lot of effort, especially if one of the terms is rare, while the
second occurs very frequently.

Based on ideas from Alexander Korotkov's fast scan patch.
Create explicit json<->jsonb casts using IO functions.
Further optimize multi-key GIN searches.

If we're skipping past a certain TID, avoid decoding posting list segments
that only contain smaller TIDs.

Extracted from Alexander Korotkov's fast scan patch, heavily modified.
Add more C comments to entab.c.
Docs tidy up. Mainly moving index terms out of tables.
Merge branch 'master' into jsonb